Prezi presentations are made on a virtual canvas, instead of slide-by-slide. You zoom in-and-out as you move from image-to-image through your presentation.
Prezi critics (disclosure: I'm one) think that the the purported benefits (the motion and in-and-out zooming) are actually distracting and annoying. Variety is always good, though, so you should make one and see how you like it. Prezi is a paid service but teachers are eligible for the free "Edu Enjoy" option.
This tutorial is the most fun. It's a Prezi (37 slides) on how to use import your existing PowerPoint slides into Prezi.
A second alternative to PowerPoint is emaze.
I first learned about emaze recently when I was at a meeting at a local university. Watching that presentation, however, I was more captivated by watching emaze's whiz-bang, eye-catching features than by the actual content of the presentation. (That's a bad thing.)
In many ways emaze offers the same benefits (like the virtual canvas) as prezi. But here's a chart (produced by emaze) listing 20 ways they think they're a better presentation alternative to either PowerPoint or Prezi, and here's a tutorial (2:15) on how to use emaze.